Polylactic acid is a biodegradable polymer with a wide range of applications. It is used in textiles, for packaging purposes, in medical applications, including surgical sutures, sustained-release capsules and drug delivery systems and in reinforcing materials for bone fractures. It is also widely processed into monoaxially and biaxially stretched films, fibers and extrusion products. Molecular weight and crystallinity of the polylactic acid polymers are significant determinants of the quality of the products formed from it. The mechanical properties of polymers are strongly dependent on the molecular weight and the crystallinity. For low molecular weight polymers, the tensile strength is negligible, and increases to relatively useful values, with increasing molecular weight and ultimately reaches as asymptotic value. Hence, polymer with low molecular weight cannot be meaningfully processed via processes such as fiber spinning and film processing. Crystallinity of poly lactic acid is an important parameter for increasing its shelf life. Highly crystalline poly lactic acids possess the lowest gas diffusivity, and so, it hinders the gas molecule from penetrating into the polymer bulk. Water vapour from the atmosphere is one such gaseous species hindered from diffusing into the polymer, consequently reducing the polymer degradation, because even very small moisture content can cause hydrolytic degradation of the polymer chains. Gas and water barrier properties are also very essential for packaging applications.
U.S. Pat. No. 5,508,378 describes a method of producing high molecular weight polylactic acid in a two step process comprising (a) melt polymerization of a lactide to obtain polylactic acid and (b) solid state polymerization of the polylactic acid, obtained in step (a) by heating at a temperature lower than the melting point of the final polymer to obtain high molecular weight polylactic acid (with average molecular weight ranging between 70,000 to 430,000 Da). In this process, residual lactides present in 5 to 56.6% by weight of the melt polymerization step participate in further polymerization in the solid state for up to 60 hours in step (b), resulting in an increase in molecular weight by 0.93 to 2.4 times.
Shinno et al. (K. Shinno, M. Miyamoto and Y Kimura Macromolecules, 30 (201) 6438-6444, 1997) disclose polymerization of L-lactide using 0.1 mol % of stannous 2-ethyl hexanoate as a catalyst. In a two-step method, melt polymerization of L-lactide is first performed at temperatures higher than the melting point of poly (L-lactide) and then postpolymerization is continued in the solid state at a temperature close to the crystallization temperature. As poly (L-lactide) crystallizes in the second stage (when the temperature is changed from 140 to 120° C.), the monomer consumption was found to reach 100%. It was observed that the molecular weight of the polymer, however, did not increase during the solid-state polymerization step.
Moon et al. (S I Moon, C Lee and Y Kimura, Polymer, 42, 5059-5062, 2001) disclose melt polymerisation of lactic acid followed by solid-state polymerization, for production of high molecular weight polylactic acid starting from lactic acid. The maximum molecular weight of 600000 Da was obtained by this process at an overall polymerization time of 32 hours. However, drastic decrease in molecular weight of the polymer has been observed after the saturation point.
The molecular weight increase in the conventional polymerization process is thus limited and even if the molecular weight increases, the time required to reach high molecular weights is high. Moreover, the crystallinity of the polymers formed has been observed to be limited, and therefore, the polymers undergo degradation easily. This could be associated with the inherent limitation of the mechanism of polymerization in which the lactide monomer units are added to the growing polymer chain.